#top-left {
    position: absolute;
    top: 0;
    left: 0;
    right: 50%;
    bottom: 50%;
    border: solid #000;
    border-width: 0 1px 1px 0;
}

#top-right {
    position: absolute;
    top: 0;
    left: 50%;
    right: 0;
    bottom: 50%;
    border: solid #000;
    border-width: 0 0 1px 0;
}

#bottom-left {
    position: absolute;
    top: 50%;
    left: 0;
    right: 50%;
    bottom: 0;
    border: solid #000;
    border-width: 0 1px 0 0;
}

#bottom-right {
    position: absolute;
    top: 50%;
    left: 50%;
    right: 0;
    bottom: 0;
    border-width: 1px 0 0 1px;
}